1. Field of the Invention
The present invention relates to a gas turbine and the combustor thereof with reduced super high frequency oscillation of combustion and with reduced emission of NOx.
2. Description of the Related Art
FIG. 6 is a longitudinal sectional view near the combustor of a gas turbine equipped with a conventional combustor. The conventional combustor will be explained here with reference to the drawing. In FIG. 6, reference numeral 101 is a combustor mounted to a rotor housing 102.
The combustor 101 has a fuel supply nozzle 103, a liner(flame tube) 104, and a tail tube 105. Reference numeral 106 is an outer casing. A bypass elbow 107 is attached to the tail tube 105. Reference numeral 108 is a bypass valve and 109 is an adjusting mechanism of the bypass valve 108.
Reference numeral 110 is an air compressor. The compressed air 111 discharged from the compressor 110 flows inside the rotor housing 102, passes around the combustor 101 as indicated by arrows, and introduced into the combustor 101 as combustion air from the upstream side of the fuel supply nozzle 103.
The combustor 101 shown in FIG. 6 is composed as described above, the fuel supplied through the fuel supply nozzles 103 is burnt, and the combustion gas is transmitted to the turbine blade part 112 to drive the turbine rotor.
The gas turbine is operated in a wide range of load and speed from start to the rated output. Therefore, it is required that the fuel is burned stable in the combustor of the gas turbine responding to the wide range of operation conditions such as air and fuel flow rate from the start to rated output.
Also, in order to reduce Nitrogen Oxide (Nox) discharged from the gas turbine combustor, a method capable of suppressing the Nox emission is strongly required.
Premixed combustion is a method of combustion to reduce the generation of NOx. In large, NOx generation increases exponentially with combustion flame temperature. By allowing the fuel to burn in a state of premixed combustion, local elevation of the combustion flame temperature can be prevented. Therefore, NOx emission can be reduced by lowering the combustion flame temperature through obtaining a lean mixture by increasing the ratio of air to fuel. Recently, to meet with the requirement for much further reduction in NOx emission, the proportion operated in lean premixed combustion is increasing.
However, in lean premixed combustion, generally a flame blowout is easy to occur as compared with diffusive combustion in which fuel burns while mixing with air, and also combustion oscillation is easy to occur. In addition, the stable operation is limited. Therefore, it is necessary to attain the reduction of NOx emission while securing stable combustion that diffusive combustion and lean premixed combustion are combined properly.
FIG. 4 shows schematically a structure of the conventional combustor in which diffusive combustion and lean premixed combustion is properly combined. Here, the structure will be briefly explained. The combustor is composed of a pilot burner 01 for diffusive combustion provided on the center axis of a substantially cylindrical flame tube(liner) 6, the pilot burner 01 being provided with a pilot fuel supply nozzle 3 having a pilot swirler vane O1a attached around the top end part of the pilot fuel supply nozzle 3; and premixed combustion burners 02 having eight main fuel supply nozzles 2a, arranged surrounding the pilot burner 01, premixing nozzles 4a, being arranged around the top end part each of the main fuel supply nozzles 2a, a disk-like nozzle plate 7 and a premixing swirler vane 5, being provided annular space between each of the main fuel supply nozzles 2a, and premixing nozzles 4a. 
In a combustor like this, pilot fuel is supplied from the pilot fuel supply nozzle 3, combustion air for burning the pilot fuel is supplied from around the pilot nozzle to effect pilot combustion which is of diffusive flame (hereunder referred to as pilot flame) in the central part of the combustor. Around the pilot flame is supplied fuel/air mixture of very high air excess ratio to be contacted with the high temperature gas of the pilot flame so as to effect main combustion composed of premixed flames (hereunder referred to as main flames).
The premixed combustion burners 02 are arranged surrounding the pilot burner 01 to allow the premixed combustion burners 02 to be located adjacent to the pilot burner 01, so the mixture spraying from the premixing nozzle 4a, mixes with the diffusive combustion flames of the pilot combustion, which dispersed by the swirling flow effected by the pilot swirler 01a, to be burned continually, the combustion air flow rate for the pilot burner 01 can be reduced, and the rate of premixed combustion can be increased resulting in reduced NOx emission.
In the drawing, reference numerals 1a and 1b show airflow, 8 shows combustion flame, 9 shows a node line of sound pressure (nodes of sound pressure: ND)).
However, with the prior art described above, combustion oscillation of very high frequency (super high-frequency combustion oscillation) which forms the acoustic mode (sound pressure mode) in the plane transversal to axis of the combustor occurs due to the coupling of the acoustic system and combustion system.
Currently, the super high-frequency combustion oscillation is suppressed by enhancing the effect of damping in the acoustic system, for example, by providing a cylindrical element with many holes 10 in the combustion zone 8 along the inner circumference of the flame tube 6, as shown in FIG. 5. However, as the cylindrical element with many holes 10 is located in the high temperature zone of combustion, design consideration of heat resistance and cooling of the cylinder is inevitable, which results in a complicated structure and increased manufacturing costs.
There are the cases the combustor has not only one node line (1ND) but a plurality of node lines of higher order of sound mode as shown in FIG. 3.
For example, FIG. 3(A) shows the case in which there are two node lines of 2nd order which partition the plane transversal to the axis of the combustor into four vibration zones of +−+− on, FIG. 3(B) shows the case in which there are three node lines of 3ird order which partition the plane into six vibration zones of +−+−+−, and FIG. 3(B) shows the case in which there are four node lines of 4th order which partition the plane into eight vibration zones of +−+−+−+−.